Bolting is the most common reinforcement of rock that is exposed to slow deformation or sudden fracture. Fundamental requirements for rock bolts are that they are to support a heavy load and to resist a large degree of strain/elongation before the bolt breaks. A large number of rock bolts introduced into boreholes and anchored in them by means of embedding in grout form together a reinforcement system that stabilizes and reinforces the rock structure during the building of tunnels, mining operations, tunneling, etc.
Several different types of bolt intended for such reinforcement systems are available. One example of a bolt for embedding in a reinforcement system is the one known as the “Kiruna bolt” or a ribbed rock bolt that consists of a rod of solid steel. Bolts of this type normally have at their forward end an anchor arrangement such as a wedge for mechanical anchoring at the bottom of the borehole, and the bolt is provided at the end that is closest to the opening of the borehole with threads for the reception of a mounting fitting, normally in the form of a washer and nut, that is placed in contact with the area of rock material that surrounds the opening of the borehole. This type of rock reinforcement system achieves its principal load-bearing ability through the adhesion along the complete length of the rock bolt that the grout gives rise to. Other types of rock bolt are also available, such as dynamic rock bolts, that differ from the ribbed rock bolt through their being provided along a central part with a sheath or similar, whereby the embedding of the bolt takes place principally along two anchoring zones, partly along a forward section of the bolt at the bottom of the borehole and partly along a section of bolt closest to the opening of the borehole.
A further type of rock bolt is a cable bolt or a wire bolt that consists of seven twisted steel threads. All of these types of bolt may be 3-10 m in length and are intended for embedding in grout.
One problem with rock reinforcement systems that consist of embedded rock bolts is that unknown fractures and natural cavities are sometimes present, which are filled by that portion of the grout that is injected into the borehole, whereby the rock bolt that is subsequently introduced into it will be insufficient embedded in the borehole, which may have the consequence that the reinforcement system is deficient and achieves a reduced load-bearing ability.
A further problem is that rock reinforcement systems that are used in fracture-rich rock are subject to heavy loads. The rock bolts may be placed under load locally at locations at which they cross large fractures between blocks, and thus subject to heavy loads that lead to the bolts becoming deformed, mainly through strain and extension. In certain cases the load exceeds the ability of the rock bolt to absorb force, such that breaks of the rock bolt arise, whereby the reinforcement system is weakened.
It is therefore important to ensure that reinforcement systems of this type truly satisfy the stringent safety requirements, both initially and in the long term.
One effective method of detecting directly the presence of a cavity in a borehole with embedded bolts is revealed by the document SE 533769. This method makes it possible to detect whether any cavities remain shortly after that hardening grout and the rock bolt have been introduced into the borehole.
SE 533769 reveals a reinforcement means, a rock bolt, that has a tube with a passage for the introduction of a medium into the borehole. The wall of the tube is equipped with radially directed holes, openings. During the use of the method and system according to SE 533769, the rock bolt is introduced into a borehole shortly after the borehole has been filled with unhardened filler material. The presence of a cavity in connection with the rock bolt is investigated by supplying a medium under pressure to the borehole through the tube. In such a case, the medium flows through the holes, the openings, to the cavity on the outer surface of the tube. This results in a measurable change in pressure that indicates that a cavity is present in connection with the rock bolt, which gives a direct indication of the initial load-bearing ability of the bolt. However, since the rock structure, and in particular rock structures that are rich in fractures, can change with time, it is of the highest importance to obtain information about the status of the bolt also after a period of time. There is, therefore, a need to be able to evaluate and monitor in a simple and reliable manner the condition of the rock bolt and its load-bearing ability at regular intervals and during a long period.